Hydraulic vane pumps are used to pump hydraulic fluid in many different types of machines for different purposes. Such machines include, for instance, earth moving, industrial and agricultural machines, waste collection vehicles, fishing trawlers, cranes, and vehicle power steering systems.
Hydraulic vane pumps typically have a housing with a chamber formed therein. A rotor is rotatably mounted in the housing. The rotor is typically of generally cylindrical shape and the chamber has a shape such that one or more rise and fall regions are formed between an outer wall of the rotor and an inner wall of the chamber. In the rise regions, a relatively large space opens between the outer wall of the rotor and the inner wall of the chamber. On the leading side of the rise region, there exists a region which is substantially a dwell, although in usual practice there exists a small amount of fall. This is sometimes called a major dwell or major dwell region. The major dwell is followed by a fall region, in which the space between the outer wall of the rotor and the inner wall of the chamber decreases. The rotor normally has a number of slots and moveable vanes are mounted in the slots. As the rotor rotates, centrifugal forces cause the vanes to move to an extended position as they pass through the rise regions. As the vanes travel along the fall regions, the vanes are forced to move to a retracted position by virtue of the rotors contacting the inner wall of the chamber as they move into a region of restricted clearance between the rotor and chamber. Hydraulic fluid lubricates the vanes and the inner wall of the chamber. Outside of the rise, fall and major dwell regions, the space between the outer wall of the rotor and the inner wall of the chamber is small. In practice, this is usually a true dwell of zero vane extension and is sometimes called the minor dwell.
Hydraulic vane pumps are usually coupled to a drive, such as to a rotating output shaft of a motor or an engine and, in the absence of expensive space invasive clutches or other disconnecting means, continue to pump hydraulic fluid as long as the motor or engine continues to operate. A rotor of the pump usually has a rotational speed determined by the rotational speed of the motor or engine.
U.S. Pat. No. 3,421,413 to Adams et al describes a sliding vane pump in which hydraulic pressure is applied to each vane in order to maintain the vanes in optimum engagement with a cam surface that encircles the rotor which carries the vanes. That patent is directed towards ensuring that the vanes remain in optimum contact with the encircling cam.
U.S. Pat. No. 3,586,466 to Erickson describes a rotary hydraulic motor having a slotted rotor and a moveable vane located in each slot. The rotor is journalled in a chamber that defines three circumferentially spaced crescent-shaped pressure chamber sections. The hydraulic motor includes a valve control means and associated passages to be able to selectively control the flow of pressurised fluid to the pressure chamber sections. This allows pressurised fluid to be supplied to one, two or all three pressure chamber sections. When pressurised fluid is delivered to all three pressure chamber sections, low speed, high torque operation occurs. When pressurised fluid is delivered to two pressure chamber sections, higher speed but lower torque operation occurs. When pressurised fluid is delivered to only one pressure chamber section, even higher speed but lower torque operation of the motor occurs.
The hydraulic motor of Erickson also includes an arrangement of passages that allow pressurised fluid to impart radially outward movement to the vanes adjacent inlet passages to the pressurised chamber sections and to impart radially inward movement to the vanes adjacent outlet passages of the pressurised chamber sections. Thus, each vane is fluid pressure urged radially outwardly into sealing engagement with the concavity or concave surface of each pressurised chamber section during initial movement of the vane circumferentially across the pressurised chamber section, the vane being moved radially inwardly by fluid pressure at the circumferentially opposite end of the pressurised chamber section, to reduce the frictional load between each vane and the inner peripheral surface portions of the chamber at areas wherein there is little or no circumferential pressure applied to the vanes (see column 4, lines 55 to 72).
The entire contents of U.S. Pat. Nos. 3,421,413 and 3,586,466 are expressly incorporated herein by cross reference.
In my co-pending International Patent Application No. PCT/AU2004/000951, I describe a hydraulic machine in which the vanes can be selectively retained in a retracted position such that the hydraulic fluid is not worked, and in which the vanes can be selectively allowed to move between the retracted position and the extended position such that the hydraulic fluid is worked by the vanes. That international patent application also describes a number of venting arrangements by which pressurised hydraulic fluid under the vanes can be vented as the vanes move into and through the fall regions. The entire contents of my International Patent Application No. PCT/AU2004/000951 are herein incorporated by cross reference.
One known limit to improving the pressure and speed capability of hydraulic fluid vane pumps is the out-of-balance forces applied to the under-vane regions in the mid quadrant. In this regard, hydraulic vane pumps typically have an inlet located at the start of the rise region (if the pump has more than one rise region, it will have more than one inlet). The inlets supply low pressure hydraulic fluid (for convenience, “hydraulic fluid” will hereinafter be referred to as “oil”) to the rise region. As the vanes move the oil through the rise region, into the major dwell and then into the tall region, the oil becomes pressurised. The pressurised oil leaves via outlets associated with each fall region of the pump.
It is also known that, in many hydraulic vane pumps, the under vane region is exposed to oil that has been pressurised to the outlet pressure. This can lead to out of balance forces being applied to the vanes. For example, when the vane is on the pressure (or outlet) quadrant, the vane is exposed to high pressure oil at both an outer tip of the vane and under the vane. Thus, the forces on the vane arising from the oil are in balance. However, in the suction (or inlet) quadrants, the tips of the vanes are exposed to low pressure inlet oil whilst the bottom of the vanes are exposed to high pressure oil. This causes an imbalance of pressure which acts to push the vanes outwardly. This force can exceed the limits of the pump specifications. If this happens, the vanes can be driven through the protective film of oil that should exist between the tips of the vanes and the pump chamber. If this occurs, damage to the vanes can be caused.
There have been some attempts to limit these forces, including:
(a) providing a small vane area over the suction quadrant to which the high pressure oil is directed and full vane area at the discharge outlet. As the force applied by the under-vane oil is a product of the oil pressure multiplied by the area over which that pressure is applied, the force is lower in the suction quadrant;
(b) pin vane arrangements which use a pin inside a separate chamber, to which high pressure oil is directed. This high pressure oil only acts on the small pin, which will typically generate insufficient force to push the vane through the oil film in the suction quadrant.
These methods are all intended to limit the under vane force in the suction quadrant. However, as the areas under the vanes in the suction quadrants to which high pressure outlet oil is directed are reduced to increase the under pressure and speed rating of the pumps, the pumps can be unstable at lower speeds and pressures as the forces are too low to hold the vanes in stable operation.
Another issue that is arising in relation to hydraulic pumps has been caused by the increasing trend to heavy vehicles (either on road or off highway) having a full stand-by system against pump line rupture or pump drive failure. In this system, there is a risk of flooding the apparatus (such as a power steering apparatus) with pressurised hydraulic fluid should the secondary or emergency pump commence operation.